1. Technical Field
The subject matter described herein relates to phase change memory devices, devices and methods for changing the state of a phase change media, and methods of storing retrievable data in a phase change memory device.
2. Description of the Related Art
Several technologies, such as magnetic random access memory (MRAM), flash, and phase change memory (PCM) have emerged to bridge the speed gap between DRAM and disc memory solutions. PCM has read latency close to DRAM, high write endurance, and is expected to have higher density than DRAM, all of which makes PCM an interesting technology for building large-scale main memory systems.
Known PCM is a type of nonvolatile memory that exploits the property of a phase change media, such as an alloy of germanium, antimony, and tellurium commonly referred to as the chalcogenide GST, to switch between two states, an amorphous state and a polycrystalline state, by applying electrical pulses which control local heat generation inside a PCM cell. The amorphous state is characterized by high electrical resistivity, whereas the polycrystalline state exhibits low resistivity. This difference in resistivity between the amorphous state and the polycrystalline state can be 3 to 5 orders of magnitude. Different heat-time profiles can be used to switch from one state to another. The proportion of states inside a PCM cell can be used to represent a variety of logical states, the simplest scenario being that of a binary cell based on the amorphous state and the crystalline state. However, the large difference in resistivity and ability to manipulate the proportion of the two states in a PCM cell has reportedly made it possible to store multiple bits per cell (MLC). It is reported that prototypes capable of storing 2 bits/cell have been proposed and some studies have also analyzed 4 bits/cell. To achieve high density, it is desirable that PCM cells store multiple bits, especially as technology scales to smaller feature sizes.
One challenge of utilizing PCM devices is to achieve reliable programming with low programming currents. Work on PCM cell designs has focused at least in part on achieving programming currents that are consistent with existing lithography generations. Because GST alloys melt at 610° C., managing heat loss is another important aspect of achieving a low programming current. One approach to managing heat loss is provided by the PCM cell illustrated in FIG. 1. FIG. 1 shows a physical structure of a PCM cell 10 often referred to as a mushroom PCM cell. Cell 10 consists of a layer of phase change media 11, a top electrode 12 above the phase change media layer and a bottom electrode 14 below the phase change media layer. Positioned between bottom electrode 14 and phase change media layer 11 is an extended vertical electrode 16 of a resistive heater material. The resistive material 16 increases in temperature in response to an electric current. The use of this embedded vertical electrode of resistive heater material serves to reduce heat loss to the silicon substrate. In known PCM cells, resistive material, bottom electrode 14 and top electrode 12 are often responsible for generation of thermal energy within the cell in response to the flow of an electric current through resistive material 16. The thermal energy generated by the electric current is transferred to a portion of the layer of phase change media schematically represented by mushroom cap shaped portion 13 where it causes an increase in the temperature of portion 13. Logically, PCM cells are described as programmable resistors. To set the PCM cell to a crystalline state, a sustained current pulse is applied to the resistive material to generate sufficient thermal energy to increase the temperature of the phase change material to above its crystallization temperature (Tc), but below its melting temperature (Ta). When heated to above its crystallization temperature, but not above its melting temperature, the phase change media takes its crystalline state having a low resistivity relative to its resistivity in its amorphous state. To reset the PCM cell to its amorphous state, a short electrical pulse is applied to the resistive material to generate thermal energy sufficient to heat portion 13 of phase change media above its melting point. Once heated above its melting point, portion 13 of phase change media layer transforms into its amorphous state (high resistivity relative to its crystalline state) which it maintains when cooled rapidly.
A thermal printer employs thermal print technology to produce a printed image by selectively heating a coated thermochromic paper when the paper passes over a thermal print-head of the thermal printer. The thermochromic coating turns black (or another color) in areas where it has been heated by the thermal print-head, thus producing an image. Two-color direct thermal printers can print both black and an additional color (often red) by providing a thermal print-head that can be heated to two different temperatures and contacted with the thermochromic paper. Such type of thermal print technology is also employed in thermal transfer printing where one side of a heat sensitive ribbon is contacted with a thermal print-head and a portion of a coating on the opposite side of the ribbon melts and attaches to a material which is in contact with the coating. Current thermal print-head technology provides resolutions as high as 4800 DPI. Print-heads capable of providing lower resolutions, such as 203 DPI, 300 DPI, and 406 DPI are commercially available. Thermal printers have been used for many applications, including producing bar code labels, clothing labels, and printing plastic labels for chemical containers.
In addition to a thermal print-head, a thermal printer includes other components, including a platen which assists in feeding a substrate to the thermal print-head, a biasing element that applies pressure to the thermal head causing it to contact the temperature-sensitive substrate or ribbon, and a controller for controlling the operation of the thermal printer. In operation, the controller includes circuits that control the flow of electrical currents to the heating elements of the thermal head, causing selected portions of the thermal head to increase in temperature. The controller may also control the speed the substrate moves relative to the print-head of the movement of the print-head relative to the substrate.
With the continued interest in PCM devices for large-scale main memory systems as well as smaller scale memory systems, interest remains in PCM cell structures that require low current to set and reset the cell.